U.S. application Ser. No. 08/730,670, entitled "Dual Mode GSM/AMPS Phones" filed Oct. 11, 1996, which is hereby incorporated by reference herein, describes various circuit architectures utilized in portable cellular telephones in which a desired transmit signal at a desired transmit frequency is produced by first modulating a transmit intermediate frequency voltage controlled oscillator (TXIF-VCO). The disclosed TXIF-VCO is preferably of the type known as a Quadrature VCO (QVCO), which produces two quadrature output signals 90.degree. out of phase with each other (Cosine and Sine). The quadrature output signals are received by a quadrature modulator, such as the improved quadrature modulator described in U.S. Pat. No. 5,530,722 issued Jun. 25, 1996, also hereby incorporated by reference herein.
The modulated TXIF-QVCO signal from the quadrature modulator is then compared in a phase comparator with a signal from a transmit final-frequency voltage controlled oscillator (TX-VCO). Prior to the comparison, the TX-VCO signal is mixed down to equal the frequency of the TXIF-QVCO signal by heterodyne mixing with a signal from a frequency synthesizer, the latter signal normally being available from the receiver where the frequency synthesizer is used as the receiver local oscillator (RXLO). An error signal is output by the phase comparator when the phases of the signals input thereto are not equal. The error signal is passed through a loop, or low pass, filter and is used to control, or tune, the TX-VCO to the desired transmit frequency, which is equal to the sum or difference of the RXLO and TXIF signal frequencies. The control loop bandwidth of the loop filter is made sufficiently wide to pass the phase modulation applied by the quadrature modulator to the TXIF-QVCO signal, and thus apply the same phase modulation to the TX-VCO, thus obtaining a modulated final-frequency transmit signal.
The above-described prior art system has been integrated into RF (Radio Frequency) integrated circuits and is used in cellular phones conforming to GSM (Global System for Mobile Communications) standards, as well as in cellular phones conforming to the PCS1900 standard.
If the TXIF-QVCO signal is a small multiple of the crystal reference oscillator frequency, as deliberately arranged in the GSM/PCS1900 circuits, the TXIF-QVCO frequency only need be frequency divided by that small multiple in order to reduce the frequency of the TXIF-QVCO signal to that of the crystal reference oscillator for phase comparison. It is well known that the phase noise of an oscillator controlled by a phase-locked loop circuit (PLL) is dependent on the frequency divider ratio, and is worse when the frequency divider ratio is large. In the aforementioned applications, the small divider ratio results in acceptably low phase noise, allowing the TXIF-QVCO to be of a multivibrator type, which may be easily integrated into a silicon integrated circuit. A suitable type of multivibrator is, for example, an emitter-coupled or source-coupled current-controlled multivibrator, based on either bipolar or field-effect transistors, respectively. An example of such a multivibrator is described in U.S. Pat. No. 5,654,677 issued Aug. 5, 1997, which is hereby incorporated by reference herein.
In other applications, it may not be possible to devise an internal frequency scheme such that the TXIF-QVCO frequency is a small multiple of the crystal reference oscillator frequency. This situation can arise when, for example, the transmitter channels are desired to be selectable with a finer step size than the receiver channels, as described in U.S. application Ser. No. 08/746,338, entitled "Satellite/Cellular Phone Using Different Channel Spacings on the Forward and Return Links" filed Nov. 8, 1996. This application and its parent, U.S. Pat. No. 5,535,432 issued Jul. 9, 1996, are hereby incorporated by reference herein. This results in a higher division ratio used in the TXIF-QVCO phase-locked loop circuit, which in turn results in higher phase noise.
The present invention is directed toward overcoming the above-mentioned problems, namely, alleviating the deleterious affects of higher TXIF-QVCO phase noise.